The production of safe drinking water from contaminated source water has been practiced for many years. Traditionally, surface water is contaminated with particulate matter from contact with the earth and microorganisms from contact with wildlife. The salinity of water is highly variable from fresh water in streams to salt water in oceans. Common sources of water contamination include agricultural, industrial, and conflict activity.
Most communities have stationary water treatment facilities designed to produce safe drinking water from the source water to the community. Natural or man-made disasters can compromise the functionality of existing water treatment facilities requiring mobile water purification equipment be deployed for disaster relief by organizations such as the Red Cross. There are also many areas that are under developed and have no water treatment infrastructure. People living and working in these areas require a mobile water purification system to treat the available water source. The military is also a large user of mobile water purification systems.
Many systems have been developed to address the need for mobile water purification systems for use on source water of unknown and variable quality. However, the success of these systems has been limited. The first mobile systems developed simply filtered and chlorinated water. These systems were ineffective for treatment of salt water or chemically contaminated water. The next generation of mobile water purification systems utilized reverse osmosis to remove dissolved salts in sea water and provide some protection against chemical contamination. But, these systems fouled very quickly when they encountered turbid water. The most advanced systems currently available continue to have fouling problems, have limited ability to remove nuclear, biological, and chemical agents, and require highly trained operational personnel.
There has been fairly extensive evaluation of the performance of the Army's “ROWPU” (reverse osmosis water purification unit). There have also been several iterations of this device with different flow rates. Despite several design changes, the “ROPWU” units remain plagued by fouling problems. Additionally, it has been recognized that for several contaminants of concern, reverse osmosis alone is not adequate to provide sufficient removal. Therefore, add on filter cartridges have been employed to increase contaminant removal capability. U.S. Army document TB MED 77 provides documentation of how to operate its “ROPWU” units for maximum effectiveness. This involves an extensive chain of command with approval of the water source to be treated following analysis of the source water, evaluation of any threat of warfare agents, and operation of the units by highly trained personnel.
All the prior art mobile water purification systems have some deficiency. Deficiencies exist in resistance to fouling, contaminant removal capability, and operator intervention requirements. Unknown source water has a high potential to foul water purification equipment as it is likely that highly turbid water will be encountered. Many systems have inadequate particulate removal capability. For example, the use of a 5 micron cartridge filter prior to reverse osmosis. The reverse osmosis membrane has a very fine pore size of less than 0.005 microns. An abundance of particulate matter resides in the range of size difference between 5 and 0.005 microns including most microorganisms, fine sand or silt, and colloidal matter. Thus, the reverse osmosis membrane is easily fouled and the foulant is difficult to remove. Biofouling is particularly difficult to remove from membranes and is well documented as a common fouling problem in membrane systems. There is also potential for oil to be present in the water source which will readily foul membrane surfaces. Furthermore, all of the prior art systems rely on operator intervention to clean the fouled systems.
There are an abundance of contaminants that may be present in an unknown source water and must be removed for the water to be safe for human consumption. The US Environmental Protection Agency and the World Health Organization have established acceptable levels for many contaminants in drinking water. The military has evaluated some contaminants and determined maximum acceptable levels for short term exposure. These appear in the Tri Service Field Water Quality Standards. Establishment of acceptable contaminant levels for drinking water is an ongoing process and standards are regularly being revised by all of these organizations.
In 1998 the U.S. Army released Medical Issues Information Papers No. 31-017 and 31-018 discussing biological warfare agents and suggesting acceptable levels of these contaminants in drinking water. As noted in these papers, extremely low concentrations of certain biotoxins are highly toxic. They recommend acceptable levels in drinking water as low as 10−5 μg/L (for Staphylococcal Enterotoxins). Therefore it is clear that water purification equipment must provide very high reduction of concentration of contaminants to be able to effectively treat potential contaminants in the source water.
The U.S. Army has also evaluated the ability of reverse osmosis to remove a number of contaminants in its Water Quality Information Paper No. IP-31-014. Some contaminants are not removed well by reverse osmosis. It is also noted that reverse osmosis membranes may become compromised. A study was presented at the American Water Works Membrane Technology Conference in 2003 that evaluated the ability of reverse osmosis membranes to remove viruses under a number of conditions including the presence of a pinhole and torn O-ring seals. Compromising of the membrane or O-ring significantly lowers the rejection ability of the reverse osmosis element.
For a mobile water purification system to be highly effective at contaminant removal, it must provide more than one pass of reverse osmosis and a means of providing adequate removal of contaminants not removed well by reverse osmosis. Previously, systems have been described that typically operate with one pass of reverse osmosis, but can be set up to run two passes of reverse osmosis. The problem with this approach is that it relies on operator intervention to convert to two pass operation and provides no means of determining if and when two pass operation should be employed. Any additional treatment for contaminants not removed well by reverse osmosis may be provided by add on cartridges but again, this relies on operator intervention to determine when and if the add on cartridges are required. Furthermore, cartridges have limited capacity and no means are provided to determine if cartridge exhaustion is provided.
Thus, there is a need for a mobile water purification system that does not require highly trained operators and pre-screening of source water. To accomplish this, the mobile water purification system should be less sensitive to fouling and capable of removal of higher levels of contaminants. Preferably, reliance on disposable filter elements that may become quickly fouled would be avoided to minimize operator intervention requirements.